The present invention relates to audio amplifiers and in particular, to digital or switching audio amplifiers, and more particularly, to an improvement of the digital or switching audio amplifier which has been designated as the Class D audio amplifier. In particular, the invention relates to what is called herein as a Class AD audio amplifier which is a hybrid amplifier comprising both a Class D and a Class A amplifier.
In a Class D amplifier, the audio information is contained within pulse width modulated signals driving an output switching amplifier circuit which provides a pulse width modulated output through a low pass filter to a loudspeaker. With reference to FIG. 1, a basic Class D audio amplifier is shown. An analog input is fed to an analog front end 10 which is an analog audio preamplifier. The analog audio signal is converted to a pulse width modulated output signal 20 by a PWM converter 25 in which the audio information is present in the duty cycle of the pulse width modulated signal. The pulse width modulated signal is then fed to a gate driver 30 which then drives the Class D audio switching stage 40 typically comprising two series-connected switches Q1 and Q2 connected in a half bridge arrangement between opposite supply voltages ±Vcc. The output is taken at the switched node Vs between the two transistors. A full bridge can also be employed. The output is a pulse width modulated signal which is fed to a low pass filter 50, i.e., an LC filter, which restores the audio information to analog form which is then provided to the load, a loudspeaker. Feedback 55 is provided from the switched node VS to the analog front end via feedback stage 60 to provide a negative feedback to control the level of the output at the desired level.
Class D amplifiers provide high efficiency and high power although they may present more distortion than a linear or Class A amplifier. In addition, Class D amplifiers have output ripple as a result of the pulse width modulation.
Class D amplifiers are very efficient but reproduction quality has suffered in the past but is improving dramatically. In contrast, linear amplifiers such as Class A and Class AB amplifiers provide low efficiency but high fidelity. Typically, Class A amplifiers are limited to a few watts RMS and Class AB amplifiers, which provide a good compromise, are practical up to the 100-200 watt range.
In the Class D amplifier, in the output LC filter, there is very little headroom between the upper audio band and the practical switching frequency which is 300-1,000 KHz typically.
The feedback loop in a Class D amplifier is taken before the output filter which requires the inductance to be fairly linear. Intermodulation distortion in the Class D amplifier is present but is often not sensed by simple single frequency total harmonic distortion (THD) measurement. Further, a residual or ripple signal at the switching frequency can be one volt or more. There is a risk of audible beats of the non-coherent ripple signal with the input.